In the art of electrostatic photocopying or photoprinting, a latent electrostatic image is generally produced by first providing a photoconductive imaging surface with a uniform electrostatic charge, e.g. by exposing the imaging surface to a charge corona. The uniform electrostatic charge is then selectively discharged by exposing it to a modulated beam of light corresponding, e.g., to an optical image of an original to be copied, thereby forming an electrostatic charge pattern on the photoconductive imaging surface, i.e. a latent electrostatic image. Depending on the nature of the photoconductive surface, the latent image may have either a positive charge (e.g. on a selenium photoconductor) or a negative charge (e.g. on a cadmium sulfide photoconductor). The latent electrostatic image can then be developed by applying to it oppositely charged pigmented toner particles, which adhere to the undischarged "print" portions of the photoconductive surface to form a toner image which is subsequently transferred by various techniques to a copy sheet (e.g. paper).
It will be understood that other methods may be employed to form an electrostatic image, such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. The charge may be formed from an array of styluses. This invention will be described in respect of office copiers, though it is to be understood that it is applicable to other uses involving electrography such as electrostatic printing.
In liquid-developed electrostatic imaging, the toner particles are generally dispersed in an insulating non-polar liquid carrier, generally an aliphatic hydrocarbon fraction, which generally has a high-volume resistivity above about 10.sup.9 ohm cm, a dielectric constant below about 3.0 and a low vapor pressure (less than 10 torr. at 25.degree. C.). The liquid developer system further comprises so-called charge directors, i.e. compounds capable of imparting to the toner l particles an electrical charge of the desired polarity and uniform magnitude so that the particles may be electrophoretically deposited on the photoconductive surface to form a toner image.
In the course of the process, liquid developer is applied to and covers the entire photoconductive imaging surface. The charged toner particles in the liquid developer migrate to the oppositely-charged areas forming the "print" portions of the latent electrostatic image, thereby forming the toner image.
Charge director molecules play an important role in the above-described developing process in view of their function of controlling the polarity and magnitude of the charge on the toner particles. The choice of a particular charge director for use in a specific liquid developer system, will depend on a comparatively large number of physical characteristics of the charge director compound, inter alia its solubility in the carrier liquid, its chargeability, its high electric field tolerance, its release properties, its time stability, etc. These characteristics are important to achieve high quality imaging, particularly when a large number of impressions are to be produced.
A wide range of charge director compounds for use in liquid-developed electrostatic imaging are known from the prior art. Pertinent examples of charge director compounds are ionic compounds, particularly metal salts of fatty acids, metal salts of sulfosuccinates, metal salts of oxyphosphates, metal salts of alkylbenzene-sulphonic acid, metal salts of aromatic carboxylic acids or sulphonic acids, as well as zwitterionic and non-ionic compounds, such as polyoxyetheylated alkylamines, lecithin, polyvinylpyrrolidone, organic acid esters of polyvalent alcohols, etc.
Most of the above-mentioned prior art charge director compounds have been used, or proposed for use, in electrostatic imaging processes, wherein the toner particles in the liquid developer system are negatively charged so that they may be electrophoretically deposited on a positively charged latent electrostatic image. Processes of the opposite type, i.e. wherein a negatively charged latent electrostatic image is produced on the photoconductive imaging surface and is developed by positively charged toner particles suspended in a liquid developer, have been less extensively used in the past, but have recently gained renewed interest. These processes will be referred to hereinafter as "positive toner processes". Such positive toner processes are described, for example, in copending U.S. patent application Ser. No. 400,715, filed Aug. 30, 1989 and entitled IMAGING ON PVC AND THE LIKE, the disclosure of which is incorporated herein by reference.
Alternatively, a positively charged photoconductor can be utilized with positive toner in a so-called reversal process, whereby the latent image is formed by removing charge from the image areas and the background areas remain charged. The development is performed with a positive developer electrode and the toner image is formed on the discharged image areas.
One of the problems encountered in such positive toner electrostatic imaging processes concerns the charge director compounds to be used in these processes. Among the wide range of prior art charge director compounds, none has yet been found which would yield fully satisfactory results when used in these positive toner processes. The main drawbacks of the charge director compounds hitherto proposed for "positive toner" processes, are the instability with time of the bulk charge of the toner particles and of the copy quality produced with liquid developer systems comprising these prior art charge director compounds. A further drawback of the prior art charge director compounds in such positive toner processes is their sensitivity to the nature of the pigments contained in the toner particles.
U.S. Pat. Nos. 3,729,419 and 3,841,893 disclosed the use of three specific organo-silicon compounds, namely vinyltriethoxysilane, gamma-glycidoxypropyltrimethoxysilane and beta-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, for use as charge directors in liquid developers including those of the "positive toner" type. However, these charge director compounds must be employed at the comparatively very high concentrations of 0.5 to 2.0% by volume in the liquid developer.
It is therefore an object of the present invention to provide charge director compounds having improved properties, particularly as regards time stability of the toner charge and copy quality, for use in liquid developed electrostatic imaging processes of the above-mentioned positive toner type.
It is another object of the present invention to provide a liquid developer system comprising the above-mentioned improved charge director compounds for use in electrostatic imaging of the positive toner type. Yet other objects of the invention will be apparent from the description which follows.